# Homework Help: Statistics question

1. Oct 18, 2006

### Rumpelstiltzkin

Hi guys, need help with my statistics assignment. I hope I'm in the correct section, the other board about statistics doesn't seem to allow homework help. Here's the question:
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7. Suppose a process is out of control and that instead of delivering items with mean μ, it is delivering items with mean > μ . We wish to determine, on average, how many samples of size 9 must be taken in order to obtain a sample with mean outside the control limits of 3 SD’s.
(a) when the mean is μ + σ
(b) when the mean is μ + 2σ
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I hate to be coming in here to ask a question without showing any working, but I really don't know how to start. I'm generally quite bad at statistics. I know I need a formula (or a few formulas) to relate the mean with the control limit. I think it has something to do with confidence intervals? Or is it something else? In any case, I don't see how I can predict the number of samples that has to be taken to get something outside the control limit. Nor do I really understand why we must deliberately take many samples until one exceeds the control limit to confirm that the process is out of control.

Any help would be very much appreciated. Thanks.